Radio and X-ray variability in the Seyfert galaxy NGC 4051

Abstract

We present intensive quasi-simultaneous X-ray and radio monitoring of the narrow line Seyfert 1 galaxy NGC 4051, over a 16 month period in 2000-2001. Observations were made with the Rossi Timing X-ray Explorer (RXTE) and the Very Large Array (VLA) at 8.4 and 4.8 GHz. In the X-ray band NGC 4051 behaves much like a Galactic black hole binary (GBH) system in a `soft-state'. In such systems, there has so far been no firm evidence for an active, radio-emitting jet like those found in `hard state' GBHs. VLBI observations of NGC 4051 show three co-linear compact components. This structure resembles the core and outer hot spots seen in powerful, jet-dominated, extragalactic radio sources and suggests the existence of a weak jet. Radio monitoring of the core of NGC 4051 is complicated by the presence of surrounding extended emission and by the changing array configurations of the VLA. Only in the A configuration is the core reasonably resolved. We have carefully removed the contaminations of the core by extended emission in the various arrays. The resulting lightcurve shows no sign of large amplitude variability (i.e. factor 50 %) over the 16 month period. Within the most sensitive configuration (A array) we see marginal evidence for radio core variability of ~25% (~0.12 mJy at 8.4GHz) on a 2-week timescale, correlated with X-ray variations. Even if the radio variations in NGC 4051 are real, the percentage variability is much less than in the X-ray band. Within the B configuration observations, where sensitivity is reduced, there is no sign of correlated X-ray/radio variability. The lack of radio variability in NGC 4051, which we commonly see in `hard state' GBHs, may be explained by orientation effects. Another possibility is that the radio emission arises from the X-ray corona, although the linear structure of the compact radio components here is hard to explain.Comment: 13 pages, 15 figures, 3 tables Accepted to MNRAS November 23. Received 2010 November 23; in original form 2010 August 2

Parsec-scale radio morphology and variability of a changing-look AGN: the case of Mrk 590

“…While the radio variability (particularly at 1.4 GHz) shows trends that are similar to that observed at optical-UV and X-ray frequencies, also suggesting an origin intrinsic to the source, these radio flux variations are marginal in comparison to the order of magnitude flux variations observed at the higher optical-UV and X-ray frequencies. Radio flux variations at similarly low (20 to 50 per cent) levels have been observed on time-scales of weeks to years in some nearby Seyfert galaxies (Mundell et al 2009), including NGC 5548 (Wrobel 2000 and NGC 4051 (Jones et al 2011). In the case of NGC 4051, the radio variability is also much lower in amplitude than the variability observed in X-rays (Jones et al 2011).…”

“…Radio flux variations at similarly low (20 to 50 per cent) levels have been observed on time-scales of weeks to years in some nearby Seyfert galaxies (Mundell et al 2009), including NGC 5548 (Wrobel 2000 and NGC 4051 (Jones et al 2011). In the case of NGC 4051, the radio variability is also much lower in amplitude than the variability observed in X-rays (Jones et al 2011). The percentage of radio variability in Mrk 590 could be higher if the variable component is responsible for only a fraction of the total flux measured by the VLA and the VLBA.…”

Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

“…Moreover, since the observations are often many years apart, the 95 GHz excess needs to be confirmed with simultaneous multi-frequency observations. Radio variability of RQ AGN has been well documented (Wrobel 2000;Barvainis et al 2005;Anderson & Ulvestad 2005;Mundell et al 2009;Jones et al 2011). Wrobel (2000 monitored NGC 5548 at 5 and 8.5 GHz and found variability of up to ∼52% on time scales of months to years, which is reflected in the larger error bars for this source in Fig.…”

“…Even less are available for simultaneous radio and X-ray observations. Recently, Jones et al (2011) published their campaign for NGC 4051. Despite the systematic monitoring, the conclusion of that study was ambiguous due to the limited radio variability at 8.4 and 4.8 GHz.…”

Water masers in NLS1 galaxies

“…This latter option seems the most favourite one given the absence of high-velocity satellite maser features (typically present in disk-masers) and being also supported by recent evidence in NGC 4051 of a weak, relatively extended ( 60 pc) jet ( [9], [10], and [12]) and of a shocked outflow ( [15] and [18]). …”